Self-tuning filter circuits for increasing ratio of narrow band variable frequency signal to broad band noise



R. P. MORK SELF-TUNING FILTER CIRCUITS FOR INCREASING Feb. 20, 1962 RATIO OF NARROW BAND VARIABLE FREQUENCY SIGNAL TO BROAD BAND NOISE Original Filed Jan. A26, 1956 35 LQ/o i 5 a 46 gwf l 39 @UTI-707' TO/P/VEY v owing to the presence of the sine wave signal. by selecting the output from the filter having the largest SELF- t G FHJIER CmCUlTS FOR DICREAS- ING RARO. F NARROW BAND VARIABLE FREQUENCY SIGNAL TO BROAD BAND NOESE Raymond P. Mark, Weston, Mass., assigner to Raytheon Company, a corporation of Delaware yContinuation of abandoned appiication Ser. No. 561,448,

Jan. Z6, 1956. IThis application duly 28, 1961, Ser. No. 129,198

(Cl. S33-17) 8 Claims.

of an audio frequency sine wave voltage and asubstantial amount of noise, which has an approximate 'uniform power-distribution over the received spectrum. Since the sine wave componentrrof this signal, called the doppler signal, isthe only part containing useful information, ,and since it occupies only a narrow band at any particular short period of time, it is possible to increase the sensitivity of the radar and generally improve its performance by passing the signal through a relatively narrow band-pass filter tuned to the frequency of the doppler signal. This filter must be tunable, however, as the frequency of the doppler signal continually varies. Manual tuning is possible but is unreliable without elaborate tuningindicators which are virtually equivalent in complexity to an automatic system.

In accordance with the present invention, an extremely simple and reliable tuning system is provided which automatically tunes itself to the strongest frequency component present in an applied signal. This result is accomplished by providing a plurality of fixed-tuned, bandpass filters, the pass bands of which just overlap to cover the entire spectrum of interest. Following the band-passl filters is a plurality of signal selectin-g circuits adapted to pass the signal from only the filter whose output voltage is largest. Operation is based on the fact that the frequency of any sine wave signal will fall within the pass band of at least one filter, and the signal will pass through that filter substantially unattenuated, while white noisel or pulse noise will have only a small amount of its power falling within the pass band of any filter, so that the noise power allowed to pass through any filter will be small. If there are enough filters, with narrow enough pass bands, a sine wave signal which is submerged in noise at the input to the filter will appear well above noise at the output of one of the filters and the output voltage of this filter will be higher than that of the others Thus,

output voltage, a signal with greatly reduced noise is obtained.

The invention will be better understood as the following description proceeds taken in conjunction with the V accompanying drawing wherein:

FIG. 1 is a schematic representation of one embodiment of a filter system in accordance with the present invention; and

FIG. 2 is a partially schematic representation of another embodiment of the present invention.

Referring now to the drawings, and more particularly to FIG. 1 thereof, there is shown a self-tuning frequency selective network comprising a plurality of narrow band- 3,022,471 Patented Feb. 20, 1962 pass filters of any desired Vtype shown generally at 1, each of which is indicated as being included within one of the dotted areas 2, 3, d, and 5, Each of the filters may be designed to pass a desired narrow band of frequencies. For example, in a successfully tested embodiment filter 2 was provided with a pass-band of 1,000 to 1,500 c.p.s., lter 3 with a pass-band of 1,500 to 2,000 c.p.s., filter 4 with a pass-band of 2,000 to 2,500 c.p.s., and filter 5 a pass-band of 2,500 to 3,000 c.p.s. It should be understood that although only four such filters are shown in FIG. l, any number of additional similar filters may be added in parallel relation to the filters shown at 2, 3, 4, and 5 in order to extend the frequency range as desired, thus obtaining very high sensitivity without increasing basic complexity.` y

As shown, the filter 2 may comprise a resistor 11 having one end connected to the secondary 6 of transformer 7, and its other end connected to the parallel resonant circuit consisting of' capacitor 12 and coil 13. One side of the resonant circuit is connected to ground while the other side is connected through capacitor 14 to one side of a second parallel resonant circuit composed of caresonant circuit also being grounded; Coil 13 and coil 16 should be so arranged that no magnetic coupling exists between them. The filter is completed by an output resistor 17 connected across the second parallel resonant circuit. Since the remaining filters 3, 4, and 5 may be similar in arrangement to that described with reference to filter 2, the only change being in the values assigned to the various components in order to provide the particular pass-band desired, it is deemed unnecessary to repeatthe foregoing description as applied to each of the filters 3, 4 and 5. For a purpose to be later explained, adjacent filters have their inputs connected to opposite ends of the secondary 6 of input transformer 7, as shown in the drawing.

Output resistor 17 is connected to a suitable rectifying element, such as to the anode 18 of a diode 19.v The cathode 20 of diode 19 is in turn connected through a smoothing network comprising resistors 21, 22, and 23, and a capacitor 24, to the anode 25 of a second diode 26 which functions as a signal-selecting element. Cathode 27 of diode 26 is connected to a common bus line 28. Output resistor 17 is tapped at a suitable point 29,

and connected through capacitor 30 to the anode 2S of the signal-selecting diode 26.

Similarly, the remaining filters 3, 4 and 5, and any additional ones which may be included, are each connected to the common bus 28 through corresponding rectifying elements 35, d5, and 55, associated smoothing networks designated generally at d0, 50 and 60, and corresponding signal-selecting elements 36, 46, and 56. Output resistors 37, 47, and 57, are also'tapped at a suitable point and connected to their corresponding signal-selecting diodes 36, 46 and 56 through coupling capacitors 38, 39 and 49, respectively.

In order to apply an incoming signal to the system, each of the filters is connected to the secondary winding 6 of an input transformer 7, the secondary 6 being center tapped to ground. Although not necessary to the invention, if signal amplication is required, it may be achieved by including the primary 8 o-f transformer '7 in the plate cirouit of a power amplifier such as a vacuum tube 9.

As previously recited, when the circuit is in operation, the frequency of any sine wave signal applied to input transformer 7 will fail within the pass-'band of at least one of the filters 2, 3, 4, and 5 and will pass through that filter unattenuated. The output voltage developed across output resistors 17, 37, 47 and 57 of each of the filters is rectified by the corresponding diode 19, 35, 45, or 5S and each signal is passed through the low-pass smoothing networkl, 40, 50 and'flfollowing'each"ofthese diodes to obtain a plurality of direct voltages proportional to rthe .average output voltage of Aeachof the, filters 2, 3, 4 and'S. `The directvoltage from each rectifying element is' appliedto itscorrespondingselectingfdiode 26, 36, 46 and 56 thefcathodes'of which arerconnected .to the cornmon busiZS, The selecting diode to which the highest ,direct voltage is applied conducts, and the cathode bus 28 rises to this voltage, thereby cutting off. the` remaining selecting diodes. As 'showmfa portion of the alternating current@signal developed across each of output resistors 17, 37, 47`a`1`1d 57 is fed through its respective "coupling capacitor 30, 38, 39'or49 to the anode of its'corresponding selecting diode`26, 36,4601." 56, and 'passes through lthe diodeftotlie' cathodebus 28 if the diode is in a conducting state, but is blocked if fthediode is cut off. A simple'resista'n'ce capacitance high pass filtercomprising ,resistonz and capacitor 33 isincluded to remove the D.C. voltage Vcomponent from lthe output signal.

Ifhe time constants of the low-pass filters 3l, 4f), 50 and .60 maybeimarde long enough to allow the selectingcirculit carrying the desired signal to remain open through a brieffade of the signal. Thenoisepassed during this .periodwillbe centered,approxiniatelyfabout the frequrencies; of Athedesiif-e :l s'ignal'before the fade,an`d the ,noise will,` therefore, serve to` give an approximate indication of the'frequencyfof the'faded 'signah The long Ytime constant in the low-pass 4filters will alsoprevent short :bursts o f interfering signals from taking over'control of the system. n l Insieme cases the frequency of the desired"rsignal may be such 4 as to produce approximately equal'outputs frorn twoadjacent'band-pass filters, both of the ycorresponding diodefselctors being open at the same time, depending 'on how, sharply the diode cuts off and on 1how high `a voltage level its used. `In this event, the signals." from the .4 two filters should be of aproper phase to add construetively, l,orJat least there should be no destructive interference. Since one signal appears near the high endof the pass-band of one of the filters, and the `other near the low wend of triepztss-band` tof the adjacent filter, theV former generally willhave alagging phase andthe latter a leading phase. These phase shifts can amount to as much as 90?, andthe signals could, cancel if the filters were driven nin the same'phase. In order to avoid this possibility, everyother filter is driven with a voltage" 180 out of phase by connecting them to opposite ends of the centertappedsecondary 6 of input transformer 7. This arrangement obviates the foregoing problem andV further ueliminates any sudden reversal in phase as the signal 50 passes one filter to the adjacent one. In FIG. 2 there is Vshown a` further embodiment of the present invention which maybe utilized wherean amplifier with automatic gain control Idesigned to hold the noise plus signal power level substantially constant at a predetermined value over a wide b and is provided ahead of the self-tuning filter. The noise poweradmitted by `the automatic gain control is divided by the narrow bandpa'ss filters into a plurality of smaller, equal-power narrow-band components. As long as the noise spectrum is flat, each of the narrow band filters (n in number) deals with into only one of then filters, this limit being approached 70 when the signal-to-noise ratio approaches infinity. An amplitude detector, such as a triode 70 having its grid 71 "connected ahead of the high pass outputlter composed ofcapacitor 43 and resistor 44,1r1ay be provided in order to give an indication of the presence or absence of a desired signal. The system may be designed so that the voltage level appearing at the output of the self-tuning filter will not be high enough to overcome the `bias voltage4 applied to normally cut-off tube 70. However, with the passage of a coherent signal through one of the narrow band filters, .tube '7u will he caused to conduct, thus energizing relay 72 to indicate the presence of the co- 'herent signal.

Although there have been described what are considered to be preferredembodiments of the present invention, various adaptations and modificationsthereof may be made withoutV departing from the spirit andscope ofthe invention as defined in the appended claims.

`What is claimed is; n

l. `A frequency-selective network comprising a signal -input section,` a.plurality of narrow band-passifilters interconnected to embrace al'predetermined spectrum com- `posed of contiguous frequency ranges, said filters being connected to saidinput section and adapted to pass a "total signal'composed of a primary signal of a variable frequency and a plurality ofjfrequency dependent noise components fof various strengths, and means connected "respectively toeamch ofsaid filters and responsive to the 'strongest frequency component of` said total signal for selectingthe' substantially lstrongest frequency component fof said totalsignalv passed b y said rfilters and passing sub- `stantially only said strongest yfrequency component of said 4to` tal. signa l to an output circuit.

2.A frequency-selective network comprising asignal aotinput section, a plurality of `parallel-connected "narrow band-pass filters interconnected to embrace apredeterfmined spectrum Vcomposed of contiguous frequency ranges, said filters being connected to said input section "and adapted to pass a total signal composed of a primary 'signalof a variable frequency and a plurality of fref quency dependent noise components of various strengths, 'and means connected respectively to each of said filters and responsive vto the strongest frequency component of ysaid total signal passed by said filters andpassing sub- 40 `stantially onlysaid strongest frequency component of said total toan output circuit,

3b. A frequency-selective network comprising a signal 4input section, a plurality of parallel-connected, fixed- `tuned narrow band-pass filters interconnected to embrace a predetermined spectrumcomposed ofcontiguous fre- 'quency ranges, said filters being connected to said input ection andadapted to pass a total signal composedof a 'primary signal 'ofa variable frequency and a plurality of frequency dependent noise components of various strengths, and means connected respectively to each of said lters and responsive to the strongest frequency com- "ponent 'of said total signal for selecting the substantially 'strongest frequency component'of said total signal passed bysaid filters and passing substantially only said strong- 'est frequency component of said totalsignal to an output 4. A frequency-selective network comprising a signal input section, da plurality of parallel-connected narrow band-pass filters interconnected to embrace a predetermined spectrum composed of contiguous frequency ranges,

each of said filters having a different and adjacent pass b and, said filters being connected to` said input section and adapted to provide frequency dependent output voltages of various strengths, and means responsive to the output voltage of the" filter passing the largest output voltage for passing "an intelligence signalto an output circuit.

5. frequency-selective network comprising a signal input section, a plurality ofparallel-connected narrow band-pass filtersinterconnected s oasto embrace a spectrurn composed of contiguous frequency ranges, each of said filters having a` different passA band and being adapted tto provide frequency dependent'output voltages of various strengths, said filters being connected to said input section, means for `splitting the output voltage of each of said filters into a directlcurrent portion and an alternating current portion, and means connected respectively to each of said filters and responsive to the substantially highest direct current voltage emanating from said iilters for passing its associated alternating current signal to an output circuit.

6. A frequency-selective network comprising a signal input section, a plurality of parallel-connected narrow band-pass filters interconnected to embrace a predetermined spectrum composed ot contiguous frequency ranges, each of said filters having different pass bands with alternate ones of said filters being connected to opposite ends of the secondary winding of an input transformer contained in said input section, rectifying means connected to each of said filters, selecting means vconnected to said rectifying means, and means connected between said rectifying means and said selecting means for passing an alternating current signal associated with the strongest rectified voltage to an output circuit.

7. A frequency-selective network comprising a signal l input section, means for establishing a predetermined filters and responsive to the substantially strongest signal emanating from said filters, and indicating means responsive to the signal passed by said selecting means and adapted to indicate the presence of said signal when it lies above said predetermined power level.

8. A frequency-selective network comprising a signal input section, a plurality of narrow band-pass filters connected to embrace a predetermined spectrum composed of contiguous frequency ranges, said iilters being connected to said input section, rectifying means connected to each of said filters vfor applying a frequency dependent direct voltage to a selecting means, a smoothing network connected between said rectifying means and said selecting means, and means connected between said filters and said selecting means for passing a portion of an alternating current signal from said iilters to said selecting means, said selecting means being responsive to the substantially highest voltage emanating from said lters for passing said alternating current signalrto an output circuit.

References Cited in the tile of this patent UNITED STATES PATENTS 1,968,460 Llewellyn July 31, 19734 2,269,011 Dallos I an. 6, 1942 2,570,431 Crosby Oct. 9, 1951 2,771,586 Di Toro Nov. 20, 1956 2,848,713 Cowart et al Aug. 19, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,022,471 February 2o, 1962 Raymond P. Mork It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 39, after signal" insert for selecting the vsubstantially strongest frequency component of said total signal Signed and sealed this 19th day of June 1962.

(SEAL) Attest:

ERNEST w. swlDER DAVID L- LADD Attesting Officer 4 Commissioner of Patents 

